New startup script setting. New dry run, check gcode switches. New system state variable. Lots of reorganizing.

(All v0.8 features installed. Still likely buggy, but now thourough
testing will need to start to squash them all. As soon as we're done,
this will be pushed to master and v0.9 development will be started.
Please report ANY issues to us so we can get this rolled out ASAP.)

- User startup script! A user can now save one (up to 5 as compile-time
option) block of g-code in EEPROM memory. This will be run everytime
Grbl resets. Mainly to be used as a way to set your preferences, like
G21, G54, etc.

- New dry run and check g-code switches. Dry run moves ALL motions at
rapids rate ignoring spindle, coolant, and dwell commands. For rapid
physical proofing of your code. The check g-code switch ignores all
motion and provides the user a way to check if there are any errors in
their program that Grbl may not like.

- Program restart! (sort of). Program restart is typically an advanced
feature that allows users to restart a program mid-stream. The check
g-code switch can perform this feature by enabling the switch at the
start of the program, and disabling it at the desired point with some
minimal changes.

- New system state variable. This state variable tracks all of the
different state processes that Grbl performs, i.e. cycle start, feed
hold, homing, etc. This is mainly for making managing of these task
easier and more clear.

- Position lost state variable. Only when homing is enabled, Grbl will
refuse to move until homing is completed and position is known. This is
mainly for safety. Otherwise, it will let users fend for themselves.

- Moved the default settings defines into config.h. The plan is to
eventually create a set of config.h's for particular as-built machines
to help users from doing it themselves.

- Moved around misc defines into .h files. And lots of other little
things.

motion_control.h

@@ -40,7 +40,7 @@ void mc_arc(float *position, float *target, float *offset, uint8_t axis_0, uint8
 // Dwell for a specific number of seconds
 void mc_dwell(float seconds);
 
-// Send the tool home (not implemented)
+// Perform homing cycle to locate machine zero. Requires limit switches.
 void mc_go_home();
 
 // Kills all motion and sets system alarm